Positron emission computed tomography apparatus and image reconstruction method

Abstract

In a positron emission computed tomography apparatus of the present invention, a calibration radiation source is rotated in a measurement space in a detector ring comprised of a lot of photon detectors to perform blank measurement, and sensitivity data are produced on the basis of photon pair detection frequencies obtained for every photon detector pair of the detector ring. Subsequently, an object is set in the measurement space to perform emission measurement, and detection pair identification signals (I, J) from a coincidence counting circuit are subjected to compensation for body motion of the object measured by a position-direction measuring section and then are converted into polar coordinate values of a detector line. At the same time, cumulative data inversely proportional to corresponding values of sensitivity data are produced on the basis of the detection pair identification signals (I, J). Then this cumulative data is cumulated at an address in an address space corresponding to the coordinate values of the detector line to be accumulated as projection data. Therefore, even with an object moving during measurement, an accurate reconstructed image can be attained by effecting body motion correction of the object and accurate sensitivity correction.

What is claimed is: 
     
       1. A positron emission computed tomography apparatus comprising: a detector ring comprised of a plurality of photon detectors arranged in ring shape around a predetermined center axis to surround a measurement space, each said photon detector detecting a photon incident thereto from said measurement space to output a photon detection signal corresponding to energy of said photon;   a rotating mechanism for relatively rotating a calibration radiation source for emitting a positron to generate a photon pair with annihilation of electron-positron pair, relative to said detector ring about the center axis of said detector ring in said measurement space;   a position-direction measuring section for measuring position and direction of an object set in said measurement space, relative to said detector ring, and outputting position-direction data corresponding to the position and direction of said object;   a coincidence counting circuit for performing energy discrimination to determine if photons detected by said detector ring are a photon pair generated with annihilation of electron-positron pair in said measurement space, based on said photon detection signals received from said detector ring, and outputting a detector pair identification signal corresponding to a photon detector pair of said detector ring each having detected the two photons constituting said photon pair;   a sensitivity data producing section for counting events of detection of photon pair for every photon detector pair of said detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal received from said coincidence counting circuit, while without setting said object in said measurement space said rotating mechanism rotates said calibration radiation source relative to said detector ring, and producing and storing sensitivity data corresponding to photon pair detection frequencies for all photon detector pairs of said detector ring;   a cumulative data producing section for producing cumulative data having values inversely proportional to values of said sensitivity data taken out from said sensitivity data producing section and, with setting said object in said measurement space, outputting said cumulative data corresponding to the photon detector pair of said detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal received from said coincidence counting circuit;   coordinate converting means for converting, based on said detector pair identification signal received from said coincidence counting circuit, distance and direction of a detector line being a straight line connecting the photon detector pair of said detector ring each having detected the two photons constituting said photon pair into coordinate values expressed by predetermined polar coordinates set in said measurement space, compensating the coordinate values of said detector line in correspondence to the position and direction of said object, based on said position-direction data received from said position-direction measuring section, and outputting coordinate data corresponding to the coordinate values of said detector line;   a projection data accumulating section for cumulating said cumulative data received from said cumulative data producing section at an address of a memory space corresponding to the coordinate values of said detector line, based on said coordinate data received from said coordinate converting means, and accumulating said cumulative data distributed in said memory space, as projection data; and   an image reconstructing section for calculating a spatial distribution of photon pair occurrence frequencies with annihilation of electron-positron pair in said object, based on said projection data taken out of said projection data accumulating section, and producing reconstructed image data corresponding to said spatial distribution of photon pair occurrence frequencies.   
     
     
       2. The positron emission computed tomography apparatus according to claim 1, further comprising an image displaying section for displaying a reconstructed image indicating the spatial distribution of photon pair occurrence frequencies with annihilation of electron-positron pair in said object, based on said reconstructed image data taken out of said image reconstructing section. 
     
     
       3. The positron emission computed tomography apparatus according to claim 1, wherein said cumulative data section produces and stores said cumulative data having the values inversely proportional to the values of said sensitivity data corresponding to all photon detector pairs of said detector ring before said detector ring detects said photon pair for said object set in said measurement space. 
     
     
       4. The positron emission computed tomography apparatus according to claim 1, wherein said cumulative data section produces and outputs said cumulative data having the values inversely proportional to the values of said sensitivity data corresponding to the photon detector pair of said detector ring each having detected the two photons constituting said photon pair every time said detector ring detects said photon pair for said object set in said measurement space. 
     
     
       5. The positron emission computed tomography apparatus according to claim 1, wherein without intentionally setting said calibration radiation source in said measurement space and with setting said object containing a positron emission nuclide therein, said cumulative data section outputs, as emission data, said cumulative data corresponding to the photon detector pair of said detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal received from said coincidence counting circuit. 
     
     
       6. The positron emission computed tomography apparatus according to claim 5, wherein with setting said object intentionally containing no positron emission nuclide in said measurement space and with rotating said calibration radiation source relative to said detector ring by said rotating mechanism, said cumulative data section outputs, as transmission data, said cumulative data corresponding to the photon detector pair of the detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal received from said coincidence counting circuit. 
     
     
       7. The positron emission computed tomography apparatus according to claim 6, wherein said projection data accumulating section cumulates said emission data and said transmission data received from said cumulative data producing section independently of each other at two types of addresses in said memory space corresponding to the coordinate values of said detector line, based on said coordinate data received from said coordinate converting means, and said image reconstructing section calculates a ratio of said emission data and said transmission data as said spatial distribution of photon pair occurrence frequencies, based on said projection data taken out of said projection data accumulating section. 
     
     
       8. An image reconstructing method of positron emission computed tomography for detecting photon pairs occurring with annihilation of electron-positron pair in a measurement space and measuring a spatial distribution of occurrence frequencies of said photon pairs by a detector ring comprised of a plurality of photon detectors arranged in ring shape around a predetermined center axis to surround the measurement space, each said photon detector detecting a photon incident thereto from said measurement space to output a photon detection signal corresponding to energy of said photon, and a coincidence counting circuit for performing energy discrimination to determine if photons detected by said detector ring are a photon pair occurring with annihilation of electron-positron pair in said measurement space, based on said photon detection signals output from said detection ring, and outputting a detector pair identification signal corresponding to a photon detector pair of said detector ring each having detected two photons constituting said photon pair, comprising: a first step of, without setting an object in said measurement space, relatively rotating a calibration radiation source for emitting a positron to generate a photon pair with annihilation of electron-position pair, relative to said detector ring about the center axis of said detector ring in said measurement space, thereafter counting events of detection of photon pair for every photon detector pair of said detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal output from said coincidence counting circuit, and producing and storing sensitivity data corresponding to photon pair detection frequencies for all photon detector pairs of said detector ring;   a second step of, with setting said object in said measurement space, measuring position and direction of said object set in said measurement space, relative to said detector ring, converting, based on said detector pair identification signal output from said coincidence counting circuit, distance and direction of a detector line being a straight line connecting the photon detector pair of said detector ring each having detected the two photons constituting said photon pair into coordinate values expressed by predetermined polar coordinates set in said measurement space, producing cumulative data having values inversely proportional to values of said sensitivity data produced in said first step, cumulating said cumulative data at an address of a memory space corresponding to the coordinate values of said detector line compensated in correspondence to the position and direction of said object, and accumulating said cumulative data distributed in said memory space, as projection data; and   a third step of calculating a spatial distribution of photon pair occurrence frequencies with annihilation of electron-positron pair in said object, based on said projection data produced in said second step, and producing reconstructed image data corresponding to said spatial distribution of photon pair occurrence frequencies.   
     
     
       9. The image reconstructing method of positron emission computed tomography according to claim 8, wherein said third step further comprising displaying a reconstructed image indicating the spatial distribution of photon pair occurrence frequencies with annihilation of electron-positron pair in said object, based on said reconstructed image data. 
     
     
       10. The image reconstructing method of positron emission computed tomography according to claim 8, wherein said second step comprises producing and storing said cumulative data having the values inversely proportional to the values of said sensitivity data corresponding to the all photon detector pairs of said detector ring before said detector ring detects said photon pair for said object set in said measurement space. 
     
     
       11. The image reconstructing method of positron emission computed tomography according to claim 8, wherein said second step comprises producing and outputting said cumulative data having the values inversely proportional to the values of said sensitivity data corresponding to the photon detector pair of said detector ring each having detected the two photons constituting said photon pair every time said detector ring detects said photon pair for said object set in said measurement space. 
     
     
       12. The image reconstructing method of positron emission computed tomography according to claim 8, wherein said second step comprises outputting, as emission data, said cumulative data corresponding to the photon detector pair of said detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal received from said coincidence counting circuit, without intentionally setting said calibration radiation source in said measurement space and with setting said object containing a positron emission nuclide therein. 
     
     
       13. The image reconstructing method of positron emission computed tomography according to claim 12, wherein said second step comprises outputting, as transmission data, said cumulative data corresponding to the photon detector pair of said detector ring each having detected the two photons constituting said photon pair, based on said detector pair identification signal received from said coincidence counting circuit, with setting said object intentionally containing no positron emission nuclide in said measurement space and with rotating said calibration radiation source relative to said detector ring. 
     
     
       14. The image reconstructing method of positron emission computed tomography according to claim 13, wherein said second step comprises cumulating said emission data and said transmission data independently of each other at two types of addresses in said memory space corresponding to the coordinate values of said detector line, based on said coordinate data, and calculating a ratio of said emission data and said transmission data as said spatial distribution of photon pair occurrence frequencies, based on said projection data.